The document discusses various sources of energy that humans use. Almost all energy comes from the sun originally, either directly through solar energy or indirectly through fossil fuels and other resources formed from sunlight. Fossil fuels like coal, oil, and natural gas are nonrenewable sources of energy formed from ancient plants and animals, and are important sources that power machines and provide electricity, but also pollute the air. Other renewable sources of energy discussed include hydropower from dams, wind power from windmills, and the importance of conserving energy from nonrenewable fossil fuels.